1. Field of the Invention
The present invention relates to an illumination apparatus, an image sensor unit, and a paper sheet distinguishing apparatus. More particularly, the present invention relates to an illumination apparatus that can emit light at a plurality of wavelengths, such as visible light and infrared light, an image sensor unit including the illumination apparatus, and a paper sheet distinguishing apparatus including the image sensor unit.
2. Description of the Related Art
Some image sensor units that read images include rod-like light guides that shape light emitted by point light sources, such as LEDs, into lines. In a configuration with a light source arranged at one end of the light guide in the longitudinal direction, the illumination strength of the light emitted from the light guide may be reduced toward the end on the opposite side. Patent Documents 1 and 2 disclose a light guide including an inclined surface at an angle that does not allow the light to transmit, the inclined surface formed at the end on the side opposite of the light source. According to the configuration, the light reaching the end on the opposite side of the light source is reflected to increase the illumination strength of the light near the end. This can make the luminance distribution of the light uniform throughout the entire light guide.
The image sensor unit that reads an image is also applied to a paper sheet distinguishing apparatus that determines authenticity of bills and the like. To determine the authenticity, the bills include regions in which images obtained under visible light and infrared light are different. Therefore, the image sensor unit applied to the paper sheet distinguishing apparatus can emit visible light and infrared light to the bills to read visible light images and infrared images of the bills.
Patent Document 1: Japanese Laid-open Patent Publication No. 2006-85975
Patent Document 2: Japanese Laid-open Patent Publication No. 2007-183470
The spectral transmittance of the light guide may be different depending on the material used for the light guide. Therefore, if the transmittance of the infrared light is lower than the transmittance of the visible light, the illumination strength of the infrared light emitted from the light guide is reduced toward the end on the opposite side, compared to the illumination strength of the visible light. Therefore, the illumination distribution of the infrared light emitted to the bills tends to be nonuniform. As a result, the reading accuracy of the infrared image may be reduced.